1. Field of the Invention
The present invention relates to an optical element, an optical system and an imaging apparatus.
2. Description of the Related Art
In an optical device such as a camera or the like, a diaphragm, a neutral density (ND) filter or the like is used in order to adjust an amount of light that enters into a lens or the like. Recently, a camera is mounted on a mobile phone, a mobile terminal or the like so that a diaphragm is used in such a camera (Patent Document 1, for example).
FIG. 1 illustrates a general diaphragm. A diaphragm 910 includes a tabular member made of a shading material and provided with an open portion 911 at a center portion. Thus, the diaphragm 910 shades light at a peripheral portion and transmits light at the center portion where the open portion 911 is formed. FIG. 1-(a) is a top view of the diaphragm 910 and FIG. 1-(b) illustrates transmittance of light along a dashed line 1A-1B in FIG. 1-(a).
Recently, the size of each camera is becoming smaller in accordance with a smaller size and a thinner size of each mobile phone or each mobile terminal. Thus, the size of each diaphragm is also becoming smaller. However, in the small-size diaphragm 910, generation of diffraction of light at the periphery of the open portion 911 cannot be ignored so that it is becoming difficult to increase resolution. This means that, a small-size diaphragm is required in which resolution is not deteriorated even with a high-pixel camera.    [Patent Document 1] Japanese Laid-open Patent Publication No. 11-231209    [Patent Document 2] Japanese Laid-open Patent Publication No. 2011-221120    [Patent Document 3] Japanese Patent No. 4,428,961
For an optical element that functions as such an optical diaphragm, as illustrated in FIG. 2, an apodizing filter is disclosed in which transmittance of light at a center portion is high and transmittance of light decreases from the center portion toward a peripheral portion. Here, FIG. 2-(a) is a top view of a diaphragm 920 that has an open portion 921 at a center portion, and FIG. 2-(b) illustrates transmittance of light along a dashed line 2A-2B in FIG. 2-(a).
Here, when forming such an optical element using a resin material, there is a case that the optical element is warped in a concave shape or in a convex shape.
As the warped optical element becomes substantially thicker, this may be a problem in particular for a mobile phone or a mobile terminal for which being smaller and thinner are required.